In the continuation of the present account, dialkyl disulphides and polysulphides are understood to mean the compounds of general formula (1):R—Sn—R′  (1)in which:                R and R′, which are identical or different, represent, each independently of one another, a linear or branched hydrocarbon radical comprising from 1 to 12 carbon atoms, preferably from 1 to 8 carbon atoms, and optionally one or more unsaturations in the form of double and/or triple bond(s);        n represents an integer between 2 and 8, preferably between 2 and 6 and more preferably between 2 and 4, limits included.        
Dialkyl disulphides are thus represented by the general formula (1) in which n is equal to 2. Dialkyl polysulphides are represented by the general formula (1) in which n is strictly greater than 2.
The dialkyl disulphides and polysulphides of formula (1) are said to be symmetrical when the R and R′ radicals are identical and are said to be asymmetrical when R and R′ are different. “Identical” is understood to mean having the same number of carbon atoms and having the same stereochemical configuration. In the context of the present invention, preference is given to dialkyl disulphides of above formula (1) (where n=2) for which R and R′, which are identical or different, represent a linear or branched hydrocarbon chain which comprises from 1 to 12 carbon atoms, preferably from 1 to 8 carbon atoms, and which is saturated (alkyl radical).
Preference is given, among the alkyl radicals which can form the R and R′ radicals, to radicals comprising from 1 to 8 carbon atoms, preferably from 1 to 6 carbon atoms, for example the methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, neopentyl and n-hexyl radicals.
In the context of the present invention, preference is given to the dialkyl disulphides chosen from dimethyl disulphide (or DMDS), diethyl disulphide (or DEDS), dipropyl disulphide (or DPDS), dibutyl disulphide (or DBDS), dipentyl disulphide (or diamyl disulphide) or dihexyl disulphide, the trisulphide homologues (n=3), tetrasulphide homologues (n=4), pentasulphide homologues (n=5) or hexasulphide homologues (n=6) of these disulphides, and their mixtures, it being understood that the propyl and butyl radicals can exist in the linear or branched form, for example n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl and tert-butyl, n-pentyl, sec-pentyl, neopentyl and others.
Among the dialkyl disulphides, dimethyl disulphide (DMDS) is a compound very widely used today in a great many industrial fields, for example chemical industries, petrochemical industries, in agrochemistry, in the building industry, and others. DMDS is used in particular as agent for the sulphurization of catalysts for the hydrotreating of petroleum feedstocks or as feedstock additive for steam cracking.
In comparison with other products used in these applications, such as commercial tert-alkyl polysulphides, DMDS exhibits numerous advantages, in particular a high sulphur content (68%) and non-coking decomposition products (CH4, H2S). Furthermore, in these applications, DMDS results in performances which are generally superior to those of other products, such as tert-alkyl polysulphides.
Among methods for the synthesis of DMDS, a particularly effective and economical method is the oxidation of methyl mercaptan by sulphur according to the reaction:

This reaction is catalysed by organic or inorganic and homogeneous or heterogeneous basic agents. This route is particularly economical when it is considered that H2S is coproduced and that the latter can be used to carry out the synthesis of methyl mercaptan by reaction with methanol according to the following reaction:

There exist other routes for the synthesis of DMDS described in the prior art, such as the oxidation of methyl mercaptan by aqueous hydrogen peroxide solution or atmospheric oxygen.
There has now been found another route for the production of DMDS and more generally of symmetrical or asymmetrical dialkyl disulphides, this other synthetic route exhibiting an economic advantage at least equivalent to that introduced by the process of the oxidation of methyl mercaptan by sulphur and in addition exhibiting the great advantage of making possible an entirely advantageous recovery in value of DSOs.
DSOs (DiSulphide Oils) are mixtures of organic disulphides produced during the treatment of the mercaptans present in petroleum fractions or gases liquefied by processes of “Merox” type (see, for example, Catal. Review—Sci. Eng., 35(4), (1993), 571-609).
These DSOs are generally and most often removed, destroyed or stored, according to different techniques, on the very site of production, in particular on gas sites, for example in sulphur recovery units. One of the techniques for destroying DSOs commonly used is that based on the Claus reaction, which makes possible a joint removal of the hydrogen sulphide (H2S) also present in crude natural gas.
The Claus reaction can be written according to the following reaction (A):2H2S+SO2→3S+2H2O,  (A)the sulphur dioxide (SO2) being prepared by oxidation of a portion of the H2S, according to the following reaction (B):H2S+3/2O2→SO2+H2O  (B)and also by oxidizing treatment of the DSOs, according to the following reaction (C), where the DSOs are illustrated by DMDS:CH3SSCH3+11/2O2→2CO2+2SO2+3H2O.  (C)
The problem is that the reactions (A) and (B) are carried out with a shortage of oxygen, whereas the reaction (C) requires an excess of oxygen, with respect to the stoichiometry, in order to be complete as the presence of unburnt carbon-based residues in the Claus reaction causes serious problems of quality of the sulphur obtained by this reaction. The amount of DSOs which it is thus possible to treat in conjunction with the H2S incineration is very limited and often less than the amount produced daily on the gas site concerned.
Patent Application FR 2 875 236 A describes very well the problems encountered with this removal technique and its alternative forms. This patent application also provides another destruction method based on the complete hydrogenolysis of these disulphides to give hydrocarbons and H2S, followed by incineration of the latter in a Claus unit.
With the aim of recovering in value rather than destroying the DSOs, International Application WO 2005/111175 provides for the use of these products as inhibitors of the formation of coke in hydrocarbon cracking furnaces. This is because it is known by a person skilled in the art that the addition of sulphur-comprising compounds (such as, for example, DMDS) to hydrocarbon feedstocks makes it possible to greatly reduce the formation of coke in the pipes of steam crackers, for example.
However, as is mentioned in International Application WO 2001/96499, DSOs are formed by oxidation of the sodium salts of the mercaptans to be removed and for this reason comprise traces of sodium in a significant amount. This has the consequence of the impossibility of a direct use, without pretreatment, in cracking furnaces as the presence of sodium modifies the metallurgy of the cracking pipes. Furthermore, the use of DSOs in another application, such as the sulphurization of catalysts for the hydrotreating of petroleum fractions, is also compromised as sodium is known to be a poison for these catalysts.
The direct recovery in value of these DSOs as synthetic intermediates is also impeded by the fact, this time, that these DSOs are mixtures of disulphides and not chemically pure molecules, such as dimethyl disulphide or diethyl disulphide, taken in isolation. For example, a DSO resulting from the treatment of a liquefied petroleum gas (LPG) comprises dimethyl disulphide, diethyl disulphide and a significant amount of ethyl methyl disulphide.
Other processes relate more specifically to symmetrical and asymmetrical disulphides. Thus, U.S. Pat. No. 2,521,870 discloses a process which makes it possible to prepare asymmetrical disulphides from a mixture of at least two different symmetrical disulphides (“disproportionation”), by heating in the presence of sodium salts. It is also known, from U.S. Pat. No. 2,557,312, to prepare symmetrical disulphides from asymmetrical disulphides (“reproportionation”) starting from a mixture of at least two asymmetrical disulphides by heating in the presence of an alkanolamine and of an alkali metal sulphide.
Furthermore, an obvious solution for a person skilled in the art desirous of producing pure, symmetrical and unmixed dialkyl disulphides from DSOs would consist in separating the various dialkyl disulphides by a distillation as a function of the boiling points. However, this technique would not make it possible to recover in value the asymmetrical dialkyl disulphides as symmetrical dialkyl disulphides, such as the ethyl methyl disulphide present in the DSOs resulting from LPG, this asymmetrical disulphide even sometimes being the predominant compound.
It is the same for more complex DSOs (complex mixtures of dialkyl disulphides resulting from condensates, for example) in which disulphides having alkyl groups of lower ranks, such as C1, C2, C3 or C4 alkyl groups, indeed even of higher ranks, are present. In these DSOs, symmetrical disulphides can be in relatively low amounts, in comparison with the asymmetrical disulphides, in the light of the various possible combinations (C1/C2, C1/C3, C1/C4, C2/C3, and the like) and the possibility of structural isomers starting from the C3 rank (isopropyl/n-propyl).